Oil and gas refineries, production units, boilers, pressure vessels, pipelines, and other operating facilities and equipment typically are designed to operate at certain pressures. An overpressure condition can occur under unusual operating conditions, such as a failure of a control valve to appropriately close or open or a failure of a controller to control temperature, pressure, or other operating parameters. Pressure relief devices, such as valves, are present in most processing facilities to limit the maximum pressure in plant equipment to safe levels. Besides relief valves, the operating facility includes other safeguards that can be considered in more accurately depicting an overpressure condition. Such safeguards, include, for example, system interlocks that lock out portions of the operating system while other portions are in use, automatic and manual shut-down controls and valves, various instrumentation that allows system overrides, and other safety items.
Discharges from multiple relief devices coupled to multiple pressure containers can be grouped into a central pipe, commonly known as a pressure relief header or a manifold. Ideally, the header is sized to accommodate multiple relief valves discharging simultaneously. The header is often discharged to an exhaust pipe where the discharge is ignited to create a “flare,” as often seen at nights in industrial sectors. Alternatively, the header can discharge to treatment facilities or other appropriate places.
The pressure relief devices are sized for various emergency contingencies that can occur in the facility. In some contingencies (called common mode scenarios), several relief devices may be required to discharge to the relief header simultaneously. Furthermore, the capacity of pressure relief devices can be adversely affected by backpressure that develops in the relief header due to flow. Backpressure is the calculated pressure downstream of the relief device. In general, the higher the backpressure, the greater the pressure increase (accumulation) in the vessel. Therefore, design engineers attempt to determine backpressures that may develop for different common mode scenarios. The ultimate goal is to ensure that the backpressure does not become too high for the relief device to protect the associated piece of equipment. The degree of risk to which apiece of equipment is exposed to can be estimated in terms of accumulation that is defined as the increase in vessel internal pressure over the vessel maximum allowable working pressure (MAWP).
In recent years, software solutions have become available for a given scenario to calculate a particular backpressure. One such program is available from Simulation Sciences, Inc. of Brea, Calif. and is known as “Visual Flare.” Such calculations have been used to determine whether a larger header should be designed and installed.
The first step in the traditional design approach is to define credible common mode scenarios based on a review of significant single failure modes (initiating events). Generally, more than one independent failure (“double jeopardy”) is not considered credible. The relative frequency of the initiating events is not considered in the evaluation. Once the credible common mode scenarios are defined, the associated relief device discharges are determined. This evaluation is typically done assuming that other protective layers present in the facility that would tend to mitigate the discharges fail to operate. As such, these “worst case” relief requirements are defined through a variety of engineering calculations. Once the discharges are identified, the backpressures in the relief header are determined directly through the use of specialized engineering software. The calculated backpressures are compared to established benchmarks to determine the acceptability of the relief header system.
A fundamental input of the software requires an estimate of how many relief valves may be discharging at any particular moment. The estimate may be based on a global estimation of the system and often is a “seat of the pants” guess at best. For safety, conservative estimates are generally made. Such conservative estimates may lead to a software solution that results in specifying a larger header than is actually needed. In some installations, the existing header may be safely used with various specific adjustments upstream or downstream of the header. A larger header or headers may cost millions of dollars to install in some facilities.
Further, a header may be appropriately sized when the facility is constructed, but inappropriately sized years later due to expansion of the facility. Typically, facilities modify their operating conditions, output, or product mix to adjust for economic conditions. However, often the header is not changed due to the attendant cost. Thus, the safety of the facility can be compromised as its ability to relieve overpressure decreases. A global estimation may indicate an overall problem, but does not indicate where to pinpoint the changes. Thus, again the known software solution may simply indicate a need for a larger header.
In the various scenarios, the relief valves and other safeguards have an individual reliability factor, that is, the probability that the individual safeguard will operate correctly at any particular time of need. The reliability factor of individual safeguards affects in some manner the overall system reliability. In addition to correctly determining multiple flow into headers, one key in an accurate solution is determining that reliability effect. The reliability coupled with flow can more accurately determine whether an unacceptable probability exist that the system will incur an unacceptable vessel accumulation condition.
To date, no known solution exists, other than the present invention, which can account for the probability of performance of the various safeguards and the effect on a system, herein including subsystems. While some software solutions evaluate an overpressure condition, many solutions are based upon a given scenario. No known solution analyzes the probability taken for the multitude of scenarios to determine the amount of risk being taken for an overall system operation.
Therefore, there remains a need for a determination of pressures and flow rates in relief pressure systems based on probabilities of various scenarios.